Such a pulse radar ranging system is known from U.S. Pat. No. 4,521,778.
Pulse radar ranging systems provide distance or level measurement based on the direct measurement of the running time of microwave pulses transmitted to and reflected from a target, e.g. the surface of a fill material in a container. As the running time for distances of a few meters is in the nanosecond range, a special time transformation procedure is required to enable these short time periods to be measured. The microwave pulses generated by a transmit pulse generator means are transmitted to the target at a repetition rate or transmit clock frequency. In a signal mixer, the received echo pulses reflected from the target are sampled by cross-correlation with sampling pulses of the same shape as the transmit pulses but at a sampling clock frequency slightly lower than the transmit clock frequency. The cross-correlation and subsequent integration or low-pass filtering leads to an intermediate frequency (IF) signal corresponding to the received echo pulses but time-expanded relative thereto by a factor T1/(T1−T2), where T1 is the transmit pulse repetition period and T2 is the sampling period. The time-expansion allows for amplifying, digitizing and further processing of the echo pulses with standard techniques.
One of the sources of errors in level measurement using pulse radar is the temperature drift. Due to variation of the parameters of the semiconductor devices of the pulse radar, the level measurement result will change over the specified temperature range of e.g. −40 to +80° C. without a real change of the measured level. The temperature drift is specified in ppm/° C. and the pulse radars on the market today generally meet +/−30 ppm/° C. specification. A reduction of the temperature drift is desirable as the error can be important for far away targets For example, at +80° C. or −40° C. and at a target distance of 20 m the error can reach 36 mm. The temperature drift is produced by changes of the timing and or shape of the transmitting and sampling pulses over temperature and changes of the time base slope (i.e. the difference between the transmit and sampling clock frequencies) over temperature.
From EP 1 770 409 A1 a pulse radar ranging system is known, where a controllable switch, depending on a control signal, either conveys the transmit pulses to the antenna to be transmitted to the target or to a calibration module, preferably a delay line of known delay and terminated with a pulse reflecting impedance mismatch.